1. Field of the Invention
This invention relates to oscillator circuits and more particularly to low-power oscillator circuits having frequency compensated amplification stages.
2. Description Of the Relevant Art
Low-power crystal oscillator circuits typically include a quartz crystal that resonates at a certain frequency and an amplifier having a single transistor gain stage for amplifying the crystal's resonant output signal. Since the single transistor gain stage has a dominant single pole in its transfer function and since only a 90 degree maximum phase shift can occur through the single transistor gain stage due to parasitic effects, the oscillator circuit is inherently stable; that is, the oscillator will only oscillate at the natural resonant frequency of the crystal. Secondary resonant frequencies are not produced as a result of the pole in the transfer function since the loop gain of the amplifier is never equal to or greater than 0 dB when a -180 degree phase shift condition exists.
Despite the inherent stability of low-power crystal oscillators having a single transistor gain stage, this type of oscillator is difficult to fabricate for operation over a wide supply range since the gain of the amplifier depends strongly upon supply current and voltage. If the supply voltage is too low, the gain of the amplifier will not be high enough to meet the drive requirements of the output signal. To solve this problem, the oscillator circuit may be modified by utilizing additional stages of amplification to increase the gain. However, this modification results in additional poles in the amplifier transfer function that can cause the amplifier to oscillate at a secondary frequency. In other words, the utilization of additional gain stages can create a condition wherein the loop gain is greater than or equal to 0 db at a frequency when the phase shift is -180 degrees. To solve this additional problem, a circuit designer can shift the gain-bandwidth of the multi-stage amplifier by means of conventional compensation such that the possibility of the secondary oscillation frequency is eliminated. A drawback to this approach is, however, that the compensation network consumes additional power. The amount of compensation desired and the power consumed by the compensation network are often directly proportional. Therefore, the more compensation that is required, the poorer the performance of the low-power oscillator.